The present invention is directed to the detection of a signal in a battery powered station, and in particular, the selection of a detector based on the detection of known symbols in a transmitted signal.
As mobile cellular stations typically operate from a battery power supply, available supply power is often limited. Therefore, during operation, minimum power consumption by the mobile cellular station is desired. Known mobile stations attempt minimum power consumption by operating using the least complex of two or more-detectors.
A detector is used in a mobile cellular station to separate information from a received signal. Some detectors are more sophisticated than others and are better able to detect the desired signal in the presence of noise, co-channel interference, or intersymbol interference. However, the more sophisticated the detector, the greater the power consumption by the detector. For example, a detector utilizing differential detection (differential detector) is adequate for detecting a signal transmitted over a channel with little or no interference (a xe2x80x9cflatxe2x80x9d channel) and consumes little power during operation. However, a detector utilizing equalization (equalizer) is required where the received signal has intersymbol interference (ISI), as in a channel subject to multipath interference, and consumes many times the power consumed by the differential detector.
A known mobile station selects a detector by assuming that the transmission channel is flat. A time slot of a received signal is stored into a buffer, and a less complex detector is used to detect the entire time slot of data. It must then be determined if the less complex detector has adequately detected the signal. To do this, the mobile station error correction decoder decodes the detected information and performs a cyclic redundancy check (CRC) on the decoded data using the CRC code. If the CRC code does not check, it is determined that the less complex detector is not adequate, and the mobile cellular station redetects the buffered signal using an equalizer. However, error correction decoding consumes approximately as much power as the equalization process performed by the equalizer.
An apparatus and method of selecting one of a plurality of detectors of differing complexity in a digital communications system using a signal having a plurality of known symbols, for example, a synchronization sequence, includes detecting the known symbols in the received signal using a less complex detector. A bit error rate (BER) is determined by comparing the detected symbols to the plurality of known symbols. The less complex detector is used if the bit error rate does not exceed a threshold, and a more complex detector is used if the bit error rate exceeds the threshold. In this way, a detector is selected using only the known symbols and without performing error correction decoding, thereby conserving power.
More specifically, where the signal is transmitted as a plurality of information packets with each information packet including the known symbols, the BER may be a running BER calculated by averaging errors of the known symbols of each information packet for a plurality of information packets.
Further, the received signal is stored in a buffer and the less complex detector detects the known symbols from the buffered signal. If the BER exceeds the BER threshold, the more complex detector is used to detect the buffered signal.
In another aspect, a Received Signal Strength Indication (RSSI) is determined for the signal. If the RSSI is less than a first RSSI threshold, the signal is rejected as no detector is adequate for detecting the signal. Further, the signal is rejected if the bit error rate exceeds the threshold and the RSSI is less than a second RSSI threshold.
In yet another aspect, the bit error rate is a first bit error rate and a second bit error rate is determined for the more complex detector wherein if the second bit error rate is not excessive, the more complex detector is used, and if the second bit error rate is excessive, bit error rates for increasingly complex detectors are determined until a detector with an acceptable bit error rate is found.
In addition, where the signal includes an accuracy verification code for essential symbols in the signal, used for determining if the essential symbols are detected and error correction decoded accurately by the receiver, the known symbols comprise a plurality of assured symbols. The assured symbols are generated by verifying the essential symbols using the accuracy verification code, and error correction reencoding the verified essential symbols. Where the cellular communications system is a time division multiple access system, the accuracy verification code is the cyclic redundancy check code.